The present application claims priority under 35 U.S.C xc2xa7 119 based upon Swiss Patent Application No. 2002 0369/02 filed on Feb. 28, 2002 which is incorporated herein by reference.
The invention concerns a method and a device for measuring the amplitude of a freely oscillating capillary of a Wire Bonder.
A Wire Bonder is a machine with which semiconductor chips are wired after mounting on a substrate. The Wire Bonder has a capillary which is clamped to the tip of a horn. The capillary serves to secure the wire to a connection point on the semiconductor chip and to a connection point on the substrate as well as to guide the wire between the two connection points. On producing the wire connection between the connection point on the semiconductor chip and the connection point on the substrate, the end of the wire protruding out of the capillary is first melted into a ball. Afterwards, the wire ball is secured to the connection point on the semiconductor chip by means of pressure and ultrasonics. In doing so, ultrasonics is applied to the horn from an ultrasonic transducer. This process is called ball bonding. The wire is then pulled through to the required length, formed into a wire loop and welded to the connection point on the substrate. This last part of the process is called wedge bonding. After securing the wire to the connection point on the substrate, the wire is torn off and the next bonding cycle can begin.
A method and a device for measuring the amplitude of the freely oscillating capillary are known from the European patent EP 498 936 B1. The measured value is used for calibrating the ultrasonic transducer. The measurement of the oscillation amplitude of the capillary is done by means of a light barrier.
A method and a device for measuring the amplitude of the freely oscillating capillary with which a laser beam is used for the light barrier are known from the Japanese patent 10-209 199.
Experiments have shown that measurement with the known technique does not provide reproducible results when the amplitude of the tip of the freely oscillating capillary is to be measured.
The object of the invention is to develop a method and a device for measuring the amplitude of the tip of a freely oscillating capillary.
The method in accordance with the invention is based on the shading of a light beam by means of the capillary whereby the oscillations of the capillary modulate the intensity of the let-through light beam. The intensity of the let-through light beam is measured by means of an opto-receiver. Generally, the direction of oscillation of the capillary in the plane is not known. However it is generally the case that the oscillations of the capillary mainly run in the direction of the longitudinal axis of the horn. With the measuring principle of the shading of a light beam, only that component of the amplitude of the oscillations of the capillary can be measured which runs perpendicularly to the direction of the light beam.
Unfortunately there are numerous undesirable effects such as, for example, contamination of the capillary, diffraction on the capillary, gradual surface variations of the capillary, asymmetries of the light beam, etc, which hamper a reproducible measurement when no special measures are taken. The invention consists in carrying out a number of measurements and averaging the measured values acquired. In a first step, the capillary is adjusted in relation to the light beam. Afterwards, the component Ay of the oscillations of the capillary which runs perpendicularly to the direction of the light beam is determined according to the following steps,:
a) Placing the capillary at one side of the light beam without it shading the light beam and applying ultrasonics to the horn;
b) Moving the capillary in a given number of n steps along a predetermined direction w where xe2x80x9cwxe2x80x9d is not necessarily perpendicular to the light beam into the light beam or completely through the light beam until it is located at the other side of the light beam whereby, with each of the i=1 to n steps, the direct voltage portion UDC(yi) and the alternating voltage portion UAC(yi) of the output signal UP(yi) of the opto-receiver are determined as well as a co-ordinate yi whereby the co-ordinate yi designates the position of the capillary in relation to a co-ordinate axis y running perpendicularly to the light beam and whereby the component Ay to be measured runs in the direction of the co-ordinate axis y;
c) Calculation of sensitivity values Si(yi) as a derivation of the direct voltage portion UDC(yi) with respect to the co-ordinate axis y as                     S        i            ⁡              (                  y          i                )              =                  ⅆ                              U            DC                    ⁡                      (                          y              i                        )                                      ⅆ        y              ;
d) Selection of at least four measurement points and, for each of these at least four measurement points, calculation of a value Ay,i as             A              y        ,        i              =                            U          AC                ⁡                  (                      y            i                    )                                      S          i                ⁡                  (                      y            i                    )                      ;
e) Calculation of the component Ay as an average of the values Ay,i.
Advantageously, the calculation of the component Ay is done by the use of statistical methods. It is of particular advantage to smooth the sensitivity values Si(yi) calculated in step c, for example by averaging over neighbouring measuring points. It is also of advantage to smooth the measured values UDC(yi) and UAC(yi). It is of further advantage to take into consideration not only four measurements but as many measurements as possible. A possible criterion for selection of the measurements is, for example, that the sensitivity Si exceeds a predetermined minimum value.